The dead zone is forecast to be between 8,500 and 9,421 square miles – an area roughly the size of New Hampshire, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). The zone is a threat to aquatic organisms as well as the humans who depend on them in the gulf’s booming seafood industry.

“Stream flows were nearly double normal during May, delivering massive amounts of nutrients to the Gulf, and that’s what drives the dead zone,” said Donald Scavia, Special Counsel to the U-M President for Sustainability and director of the Graham Sustainability Institute.(See pictures of flood waters rising along the Mississippi river.)

Scavia noted that the most likely 2011 scenario is a Gulf dead zone of at least 8,500 square miles. This estimate far surpasses the 6,000-square-mile average of the past five years, as well as the current record, set in 2002, of 8400 miles.

The oxygen-starved Gulf dead zone is largely caused by farmland runoff containing fertilizers and livestock waste from as far away as the Corn Belt. Nitrogen and phosphorus from these sources flow down the Mississippi River and into the Gulf in late spring and summer each year, prompting explosive algal blooms, which later die and sink to the ocean floor. As they decompose, the algae provide bottom-dwelling bacteria with organic matter to feast on. Oxygen is consumed in the process, producing an oxygen-starved region in bottom and near-bottom waters: a dead zone.

This year, nitrogen and phosphorus have been seeping from Mississippi and Atchafalaya Rivers into the Gulf in alarmingly high amounts. In May 2011, 164,000 metric tons of nitrogen were transported to the northern Gulf, according to the U.S. Geological Survey – a 35% climb from average May nitrogen estimates in the last 32 years. The Gulf has seen a shocking 300% increase in nitrogen content since 1960.(See the top 10 historic U.S. floods.)

Increased stream-flow rates and agricultural runoff are the main culprits, Scavia said.

“Yes, the floodwaters really matter, but the fact that there’s so much more nitrogen in the system now than there was back in the ’60s is the real issue,” he explained.

Scavia called the growth of the Gulf dead zones an “ecological time bomb.”

“Without determined local, regional and national efforts to control them, we are putting major fisheries at risk,” he said.

Gulf fisheries recorded a high dockside value of $629 million in 2009, and nearly 3 million recreational fishers – who took 22 million fishing trips – contributed a further $1 billion to the Gulf economy. The Gulf of Mexico/Mississippi River Watershed Nutrient Task Force aims to reduce the size of the dead zone to around 1,900 square miles.(See “Coastal Dead Zones Are Growing.”)

Nutrient load models can be complicated by short-term weather patterns moving water masses or mixing up the water column, say researchers at Louisiana State University.

But that doesn’t mean we shouldn’t be worried. “While there is some uncertainty regarding the size, position and timing of this year’s hypoxic zone in the Gulf, the forecast models are in overall agreement that hypoxia will be larger than we have typically seen in recent years,” NOAA Administrator Jane Lubchenco said in a statement.

Gulf Restoration Network Director of Science and Water Policy Matt Rota is frustrated with the EPA’s handling of the expanding dead zone, which he calls “another harsh reminder that our country must work aggressively to clean up the Mississippi River.”

“The EPA must stop dragging its feet in addressing the Dead Zone,” he said, explaining that the Gulf Restoration Network petitioned EPA to address the Dead Zone in 2008. “Almost three years later, the EPA still hasn’t responded to this petition, and the Dead Zone continues to plague the Gulf impacting wildlife and coastal communities.”

The actual—as opposed to estimated—size of the 2011 Gulf dead zone will be released after a NOAA-supported monitoring survey led by the Louisiana Universities Marine Consortium between July 25 and August 2.